Scintillators are widely used detectors for spectroscopy of energetic photons (X-rays and gamma-rays). These detectors are commonly used in nuclear and high energy physics research, medical imaging, diffraction, non-destructive testing, nuclear treaty verification and safeguards, and geological exploration. Important properties for the scintillation crystals used in these applications include high light output, high gamma ray stopping efficiency (attenuation), fast response, low cost, good proportionality, and minimal afterglow. There is continued interest in the search for scintillator materials that have these properties.
One form of medical imaging is called positron emission tomography and is better known by its acronym PET. PET is a functional imaging technique with the potential to quantify the rates of biological processes in vivo. PET imaging can provide diagnosis for symptoms of diseases such as cancer, Alzheimer's disease, head trauma, and stroke.
In PET (or PET scan), the patient is injected with a molecule labeled with a positron-emitting radioactive element. In some applications the radiotracer is distributed through the body, and concentrated in (or excluded from) target tissues of interest. The radioactive material decays by emission of a positron, or antiparticle of the negatively-charged electron. The positron is slowed down within a short distance from the emission point and forms a short-lived “atom” consisting of the positron and an electron from a nearby atom. The “atom,” referred to as positronium, decays by the annihilation of its constituents. This annihilation produces two essentially back to back gamma rays. The gamma rays can be detected, for example by scintillator-based detectors surrounding the body, and an image can be generated.